A new study finds that 70 percent of coral reefs worldwide will be damaged by warmer and more acidic waters

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Coral bleaching off Reunion Island in the Indian Ocean. Image via Wikimedia Commons/Elapied

Most concerns when it comes to rising greenhouse gas emissions involve changes to aspects of climate: warmer air temperatures, erratic weather patterns and the impacts of these trends on landscapes and agriculture. One of the most immediate dangers to the environment, though, is a drastic change to the chemistry of an ecosystem that covers 71 percent of the planet but many of us rarely see—the ocean.

As we covered previously, higher concentrations of carbon dioxide in the atmosphere result in an increasingly acidic ocean, as roughly a third of the carbon dioxide we emit annually (35 billion metric tons) diffuses into the surface layer of water and is converted into carbonic acid. Scientists have long known that a more acidic ocean poses grave problems for wildlife, especially for creatures associated with coral reefs, which are home to one quarter of all species of life in the oceans.

Scientists have not only been studying how acidic and warmer waters harm ocean life but also how quickly that damage is happening, and they can now put a number on the extent of the potential damage: At least 70 percent of coral reefs are projected to suffer from degradation by 2030 without a dramatic change in how much carbon we emit, according to a study published yesterday in Nature Climate Change. Scientists from the Potsdam Institute for Climate Impact Research and elsewhere arrived at this number by conducting the first comprehensive global survey of the impact of both acidification and climate change on coral reefs.

“Our findings show that under current assumptions regarding thermal sensitivity, coral reefs might no longer be prominent coastal ecosystems if global mean temperatures actually exceed 2 degrees Celsius above the pre-industrial level,” says lead author Katja Frieler of the Potsdam Institute. Many prominent climatologists now believe that there is “little to no chance” of avoiding a 2 degrees Celsius (3.5 degrees Fahrenheit) increase and view it as a realistic best-case scenario even if we begin curtailing greenhouse gas emissions immediately.

To calculate just how many reefs would be damaged by climate change and acidification, the researchers looked at 2,160 coral reefs around the globe and modeled what would happen to them under a number of different greenhouse gas emissions scenarios, ranging from the most optimistic to the most dire. They used 19 separate climate models, simulating 32,000 years in total, allowing for the widest possible range of outcomes.

The 70 percent figure might seem dire, but even that occurred under what the researchers termed “an ambitious mitigation scenario” for greenhouse gas emissions. Without any mitigation—a “plan” we’re currently pursuing—they found that all reefs would be subject to degradation. The reason for this staggering degree of damage is that corals are doubly harmed by greenhouse gases—they’re severely affected by both warmer waters (an effect of climate change as a whole) and ocean acidification.

Warming harms coral reefs through a process called bleaching. Reefs are actually inert physical structures created by living animals called corals, which in turn obtain most of their energy through a symbiotic relationship with microscopic algae. This symbiotic process, though, breaks down in the presence of unusually warm waters, causing the corals to die and the reefs to bleach, turning a pale white color. Although corals can survive brief periods of warm water, extended heat stress kills them en masse, something seen in 1998, when a prolonged period of unusual warmth tied to El Niño killed an estimated 16 percent of shallow-water reefs worldwide.

When coral reefs are already stressed by bleaching, acidification can be deadly. Just as shellfish use naturally-occurring calcium carbonate in the water to construct their shells, corals use it to build reefs, their external skeletons. More acidic water, though, directly reduces the amount of calcium carbonate available for construction. Most previous models for estimating potential damage to reefs by warming, the researchers say, neglected to take in the exacerbating damage done by acidification.

Furthermore, the particular life traits of corals make it much less likely that they will be able to adapt to warmer and more acidic waters. “Corals themselves have all the wrong characteristics to be able to rapidly evolve new thermal tolerances,” says co-author Ove Hoegh-Guldberg, a marine biologist at the University of Queensland in Australia. “They have long life cycles of five to 100 years and they show low levels of diversity due to the fact that corals can reproduce by cloning themselves.” This means that advantageous traits that would allow them to tolerate the conditions they’ll face in the future are much less likely to spread within the timeframe necessary.

All this is likely to be gone within decades, though, if we don’t quickly change our carb0n emission habits. “The window of opportunity to preserve the majority of coral reefs, part of the world’s natural heritage, is small,” says study co-author Malte Meinshausen of the Postdam Institute. “We close this window if we follow another decade of ballooning global greenhouse-gas emissions.”

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